perf: replace string concatenation with arithmetic in DecimalType.serialize

[mykaul]

Summary

• Replace int(''.join([str(d) for d in digits])) with a multiply-and-add loop in DecimalType.serialize()
• Eliminates per-digit str() allocations and the intermediate joined string

Details

Decimal.as_tuple() returns digits as a tuple of ints (0-9). The previous code converted each digit to a string, joined them, and parsed the result back to int. The new code uses simple arithmetic: n = n * 10 + digit.

Before

unscaled = int(''.join([str(digit) for digit in digits]))

After

unscaled = 0
for digit in digits:
    unscaled = unscaled * 10 + digit

Benchmark results (isolated digit-to-integer conversion)

Digit count	String join (us)	Arithmetic (us)	Speedup
3	0.215	0.081	2.66x
20	0.900	0.582	1.55x
31	1.296	0.964	1.34x

Benchmark script included at benchmarks/decimal_serialize.py.

Testing

• All 651 existing unit tests pass (16 pre-existing skips)
• Correctness verified via round-trip serialize/deserialize for small, medium, and large Decimal values